This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Light is a fundamental signal that regulates important physiological processes such as development and circadian rhythm in living organisms. Phytochromes form a major family of photoreceptors responsible for red light perception in plants fungi and bacteria1. They undergo reversible photoconversion between red-absorbing (Pr) and far-red-absorbing (Pfr) states thereby ultimately converting a light signal into a distinct biological signal that mediates subsequent cellular responses. Our research aims to understand the molecular mechanisms of Pr/Pfr photoconversion and signal transduction in phytochromes using representative bacteriophytochromes (BphP) from P. aeruginosa and R. palustris. BphPs consist of three N-terminal photosensory domains (PAS GAF and PHY) and a C-terminal histidine kinase (HK) effector domain in which the HK activity is regulated by light signals detected by the photosensory core domains (PCD). During previous proposal periods we have determined several crystal structures of PCD in the Pr and Pfr states respectively. We have also captured three early reaction intermediates by initiating and following the Pfr-to-Pr photoreaction in photoactive PaBphP crystals using temperature-scanning cryo-crystallography. Taken together these crystal structures of reactant product and intermediate states provide structural insight into initial molecular events of BphPs upon sensing red-light and subsequent conformational changes in the chromophore and adjacent protein matrix. During the next period (1) we plan to extend our crystallographic studies to full-length (FL) BphPs which include a C-terminal hisitidine kinase (HK) effector domain. We aim to identify tertiary and quaternary structural elements that are responsible for transmitting light-induced local conformational changes in the chromophore-binding pocket to the remote HK to generate a biological signal. (2) We will further explore time-resolved experimental strategies based on both Laue and monochromatic diffraction to study reaction intermediates on various time-scales (from ps to ms) in collaboration with beamline scientists at BioCARS. (3) We will carry out crystallographic studies on artificial photoreceptors if suitable projects/crystals arise in couple with our ongoing protein design efforts to convey light-sensitivity to otherwise light-inert systems such as ion channels.